1. Field of the Invention
The present invention relates to a piston assembly for use in an internal or external combustion engine. More specifically, the present invention relates to a piston assembly having a metallic body and ceramic cap.
2. Description of the Prior Art
In some combustion engines it is desirable to use pistons having a cap portion composed of ceramic and a body or skirt portion composed of metal. Pistons of this type have several advantages over pistons composed entirely of metal, with the major advantage being that they permit operation at elevated temperatures.
An example of a type of engine which can benefit from the use of a ceramic piston cap is a supercharged engine. The output of some engines can be greatly increased through the use of a supercharger. However, such engines stress conventional pistons to their design limit due to increased combustion pressure and a consequent heat build-up in the piston. In extreme cases this heat build-up results in a deformed or cracked piston skirt. Such deformation and cracking can be prevented by means of a ceramic cap placed at the head of the piston to insulate the piston skirt from the extreme heat generated in the engine chamber. Because such a ceramic cap insulates the piston skirt from much of this heat, the metallic piston skirt operates at a lower temperature than would be the case if it were integral with a metallic piston head. The lower skirt temperature permits closer tolerances between the piston skirt and the engine chamber, thereby reducing noise generated by the piston during operation.
One method of attaching a ceramic piston cap to a metallic piston body is by means of bolts as illustrated in prior art FIG. 1. This method of attachment causes heat loss from the ceramic cap to the metallic body via the bolts, as well as increased pressure sensitivity at the bolt/cap interface. This method eventually results in bolt failure due to melting, corrosion or mechanical failure of the bolts at the elevated temperature allowed by use of the ceramic cap, as well as cracking of the ceramic cap due to stress concentration at the bolt heads. A second method of attachment is by means of a cap extension which is secured directly to the piston pin, as illustrated in prior art FIG. 2. This method requires a fixed amount of space between the cap extension and the pin to allow for the different rates of thermal expansion between metal and ceramic. Such a space produces excessive movement between the ceramic and metallic pieces, thereby increasing unwanted noise produced by the piston in operation. Moreover, tensile stress induced by differential thermal expansion of the cap extension and piston pin during piston operation leads to mechanical failure of the cap extension. Devices using similar connection methods, but not employing ceramic caps, are shown in U.S. Pat. Nos. 2,310,907 to Hall and 4,013,057 to Guenther, respectively.
Other methods of attachment include casting the cap into a preformed metallic skirt (U.S. Pat. No. 2,473,254), or securing the cap to the skirt by means of matching grooves (U.S. Pat. Nos. 2,257,236; 1,969,470; 1,743,323) or threads (U.S. Pat. No. 1,357,851). Each of these methods ultimately results in mechanical failure of the ceramic cap due to the pressure sensitivity of brittle ceramic and the thermal expansion differential between ceramic and metal.
An objective of the present invention is to provide a ceramic capped piston capable of operation in high output engines. Another objective of the present invention is to allow sufficient space for differential thermal expansion between the ceramic cap and the metallic body of a ceramic capped piston, while at the same time preventing excessive movement between the ceramic and metallic pieces. A further objective of the present invention is to distribute stresses between the ceramic and metallic pieces of a ceramic capped piston sufficiently to prevent mechanical failure of the ceramic during operation.